Method and Apparatus of Breathing-Controlled Electrical Stimulation for Skeletal Muscles

ABSTRACT

Methods and devices are provided such that electrical stimulation can be delivered to a patient&#39;s skeletal muscles in response to certain respiratory signals, such as when voluntary breathing is detected.

GRANT SUPPORT DISCLOSURE

This invention was made, at least in part, with funding from theNational Institutes of Health (1R15NS053442-01A1). Accordingly, theUnited States Government has certain rights in this invention.

FIELD OF THE INVENTION

The present invention relates generally to the electrical stimulation ofskeletal muscles, and more particularly to a method and apparatus ofbreathing-controlled electrical stimulation of skeletal muscles.

BACKGROUND OF THE INVENTION

Neuromuscular electrical stimulation is well known in the art and isused for a variety of clinical applications, including as a method forstrengthening skeletal muscles. Typically, a conventional protocol ofpreset frequency, duration and intensity of electrical stimulation isprescribed to a patient (Stackhouse 2008). Electrical stimulation,particularly electromyogram (EMG)-triggered neuromuscular stimulation,has been used for many years in clinical settings to facilitatepost-stroke motor recovery of finger extension impairments (Heckmann etal. 1997; Chae et al. 1998; Francisco et al. 1998; Cauraugh et al. 2000;Crisan and Garner 2001; Cauraugh and Kim 2002; Bocker and Smolenski2003; Chae 2003; Bolton et al. 2004; de Kroon et al. 2005).

Roughly one third of all patients who experience a stroke have someresidual impairment of the upper extremity (Parker et al. 1986; Gray etal. 1990; Nakayama et al. 1994), with a major impairment being of handfunction (Trombly 1989). Common post-stroke hand function impairmentsinclude a stereotypically flexed resting posture of the wrist andfingers and an inability to extend fingers voluntarily. According to theliterature, the contributing factors include biomechanical alterations,such as muscle atrophy (O'Dwyer et al. 1996); contractures (Metoki etal. 2003) and increased muscle stiffness (Dietz et al. 1991; Ibrahim etal. 1993); and neurological changes, such as wrist and finger flexorhypertonia—spasticity (Powers et al. 1988; Powers et al. 1989; Thilmannet al. 1991; Kamper and Rymer 2000; Kamper and Rymer 2001; Kamper et al.2003), excessive coactivation of flexors and extensors (Hammond et al.1988; Dewald et al. 1995), and reduced reciprocal inhibition (Nakashimaet al. 1989; Baykousheva-Mateva and Mandaliev 1994). A recent studyassessed the relative contributions of these mechanisms to overallfinger and hand impairment in chronic hemiparetic stroke survivors andfound that weakness in grip strength and finger extension strengthaccounted for the greatest portion of deficits in hand motor controlafter stroke (Kamper et al. 2006). As such, strengthening of finger(wrist) extensors and spasticity reduction in finger flexors areextremely important for improvement of hand function in stroke patients.

The EMG-triggered electrical stimulation protocol involves initiation bythe patient of a voluntary contraction for a specific movement until themuscle activity (as measured by EMG) reaches a threshold level. When themuscle activity reaches the threshold level, it triggers an electricalstimulus to the target muscles, which facilitates the patient'smovements (Heckmann et al. 1997; Chae et al. 1998; Francisco et al.1998; Cauraugh et al. 2000; Crisan and Garner 2001; Cauraugh and Kim2002; Bocker and Smolenski 2003; Chae 2003; Bolton et al. 2004; de Kroonet al. 2005). This intervention protocol has been found superior topassive neuromuscular stimulation in motor recovery, most likely due tothe active engagement of patients during electrical stimulation therapy(Chae and Yu 2000; Chae 2003; Bolton et al. 2004; Kimberley et al.2004). Electrical stimulation has been shown to produce immediatereductions in spasticity that may last from minutes to a few hours(Dewald et al. 1996). It has been reported that long-term users (greaterthan 16 months) may have longer lasting reductions in spasticity(Apkarian and Naumann 1991). However, the effectiveness of currentelectrical stimulation techniques on spasticity reduction remainscontroversial (Stackhouse 2008).

Further, the use of EMG-triggered electrical stimulation is associatedwith a few disadvantages, including in the finger/wrist rehabilitationcontext. First, EMG-triggered electrical stimulation requires voluntaryactivation of the muscles, by finger/wrist extension, to a certainpreset threshold level. This requirement limits its application,especially for patients with moderately to severely impaired fingerextension. Second, it requires “clean” EMG signals from the targetedmuscle(s). This may be problematic when surface EMG signals areutilized. Though improved by using intramuscular EMG signals, thistechnique imposes other problems, including convenience, compliance andcost. Lastly and most importantly, inappropriate coactivation of fingerflexors and extensors may cause serious problems when using assistedelectrical stimulation (Kamper and Rymer 2001). For example, whenpatients try to assist the stimulation to the finger extensors, handopening is significantly reduced due to inappropriate coactivation(Kamper and Rymer 2001) and finger flexor hypertonia (Chae and Hart2003). Kamper and Rymer observed that attempts of voluntarymetacarpao-phalangeal (MCP) joint extension actually resulted in MCPjoint flexion in some hemiparetic patients (Kamper and Rymer 2001).

The clinical applications of EMG-triggered electrical stimulation forwrist/finger motor recovery are thus limited, and similar drawbacks tothose discussed above are associated with EMG-triggered electricalstimulation in the other applications for which it is available.Moreover, as noted, passive neuromuscular stimulation has been foundinferior to EMG-triggered stimulation for motor recovery.

Therefore, it would be desirable to have an improved system fordelivering electrical stimulation to skeletal muscles.

SUMMARY OF THE INVENTION

A breathing-controlled electrical stimulation device and methods of useare provided, such that delivery of electrical stimulation to targetmuscles can be synchronized with a patient's voluntary breathing to takeadvantage of the discovered coupling between voluntary breathing andnon-respiratory skeletal muscles.

The invention provides improved methods and devices for synchronizingneuromuscular electrical stimulation with voluntary breathing. Oneobject of the present invention is to provide methods and devices whichenable the use of neuromuscular electrical stimulation for a broaderrange of clinical applications. Another object of the present inventionis to provide methods and devices which enhance the effectiveness ofelectrical stimulation to skeletal muscles. Another object of thepresent invention is to provide methods and devices which avoid theexcessive coactivation problems associated with prior art electricalstimulation systems. The invention particularly provides methods anddevices which enhance the effectiveness of motor recovery for strokepatients, based on the finger flexion-expiration, fingerextension-inspiration coupling (Li & Laskin 2006). These and many otheradvantages and features of the invention will become apparent to thoseskilled in the art upon reading the present specification of thepreferred embodiments.

In one aspect, methods for breathing-controlled electrical stimulationare provided. The method can comprise collecting a patient's respiratorysignal; interpreting the respiratory signal to detect at least onerespiratory parameter capable of differentiating voluntary breathingfrom autonomic breathing; and delivering electrical stimulation to apatient's target muscles when voluntary breathing is detected.Alternatively, the method of breathing-controlled electrical stimulationcan comprise collecting a patient's respiratory signal; interpreting therespiratory signal to detect the value of at least one respiratoryparameter which is capable of differentiating voluntary breathing fromautonomic breathing; comparing the detected value of the at least onerespiratory parameter with a preset threshold value; and deliveringelectrical stimulation to a patient's muscles when the detected valuemeets the preset threshold value. In certain embodiments of thesemethods, the detected respiratory parameter is airflow rate. In otherembodiments of the methods, electrical stimulation is delivered onlywhen the detected respiratory parameter indicates forced inspiration. Infurther embodiments, electrical stimulation is delivered to thepatient's muscles through surface electrodes. In preferred embodiments,the patient's target muscles which receive electrical stimulation are inthe arms or legs. In additional preferred embodiments, the patient'starget muscles are in the wrists or fingers.

The methods can be used for rehabilitation following a stroke. Themethods can also be used for rehabilitation following a traumatic braininjury. In further embodiments, the methods can be used in patientpopulations with different neurological disorders, such as spinal cordinjury, cerebral palsy, or multiple sclerosis. In addition, the methodscan be used in healthy individuals for performance enhancement.

In another aspect, a device for breathing-controlled electricalstimulation is provided. The device includes a respiratory signalcollector adapted to receive a respiratory signal; a respiratory signalmonitor adapted to interpret the respiratory signal to detect the valueof at least one respiratory parameter; a control circuit adapted tocompare the detected value of the at least one respiratory parametervalue to a preset threshold value, further wherein the control circuitsends a trigger command when the detected value reaches the thresholdvalue; and an electrical stimulation delivery controller adaptedgenerate electrical stimulation upon receipt of the trigger command fromthe control circuit. In one embodiment, the respiratory signal iscollected by a face mask. In one embodiment, the detected respiratoryparameter can differentiate voluntary breathing from autonomicbreathing. In a preferred embodiment, the detected respiratory parameteris airflow rate. In a further embodiment, airflow rate is one ofmultiple respiratory parameters detected by the device.

The device can include surface electrodes which are adapted to deliverelectrical stimulation to a patient's skeletal muscles. Alternatively,the device can include implantable electrodes which are adapted todeliver electrical stimulation to a patient's skeletal muscles. In oneembodiment, a single electrical pulse is delivered to the patient'starget muscles corresponding to each trigger command. In anotherembodiment, a series of electrical pulses is delivered to the patient'starget muscles corresponding to each trigger command. In certainembodiments of the device, the patient's target muscles which receiveelectrical stimulation are in the arms or legs. In another preferredembodiment, the patient's target muscles are in the wrists or fingers.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages of the present invention will be betterunderstood and more readily apparent when considered in conjunction withthe following detailed description and accompanying drawings whichillustrate, by way of example, preferred embodiments of thebreathing-controlled electrical stimulation system and in which:

FIG. 1 is an illustration of one embodiment of the breathing-controlledelectrical stimulation system.

FIG. 2 is a block diagram showing functional components of oneembodiment of the breathing-controlled electrical stimulation system.

FIG. 3 is a graph showing example respiratory cycles.

FIG. 4 is a flowchart showing one embodiment of an algorithm employed bythe control circuit component of the breathing-controlled electricalstimulation system.

FIG. 5 is a series of graphs showing a stroke patient's responses duringa study of electrical stimulation during different breathing conditions.

FIG. 6 is a series of graphs showing a stroke patient's responses toelectrical stimulation before and after a 25 minute session using oneembodiment of the breathing-controlled electrical stimulation system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

As required, detailed embodiments of the present invention are disclosedherein; however, it is to be understood that the disclosed embodimentsare merely exemplary of the invention which may be embodied in variousforms. Therefore, specific structural and functional details disclosedherein are not to be interpreted as limiting, but merely as a basis forthe claims and as a representative basis for teaching one skilled in theart to variously employ the present invention in virtually anyappropriately detailed structure.

Recent research has led to the discovery of the phenomenon ofrespiratory-motor coupling during voluntary breathing. Voluntarybreathing, or forced inhalation and exhalation, is different fromautomatic breathing. Automatic control of breathing occurs at the brainstem level through activation of the cortical respiratory center. Duringvoluntary breathing, humans voluntarily suppress this automatic controlof breathing. Voluntary breathing is associated with cortical activationwithin the primary motor cortex. The use of non-respiratory skeletalmuscles, for example finger flexors/extensors and arm and leg muscles,is also associated with cortical activation, in the motor cortical areasin particular. The two types of cortical activation are distinctlydifferent, indicating that non-respiratory muscles are not related torespiratory activities. As such, the breathing-associated corticalactivation in the primary motor cortex could act in concert directly orindirectly with the descending motor drive from the primary motor cortexto the non-respiratory skeletal muscles.

Respiratory-motor coupling has been observed in both large and smallmuscle groups (Li and Laskin 2006; Li and Yasuda 2007; Ikeda et al. inpress). In particular, it has been reported that there is a couplingbetween finger flexion and expiration, and between finger extension andinspiration (Li and Laskin 2006; Li et al. 2007; Li and Yasuda 2007).According to this finger flexion-expiration, fingerextension-inspiration coupling, forceful inspiration facilitates fingerextension and forceful expiration facilitates finger flexion duringvoluntary breathing. Studies using transcranial magnetic stimulation(TMS) (Hartley et al. 2008) and electrical stimulation (Li et al. 2007;Sieler et al. 2008) have provided evidence that the coupling is largelymediated by intrinsic neurophysiological mechanisms. Taken together, theperformance and function of non-respiratory skeletal muscles isinfluenced during voluntary breathing, largely via activation of thecortical respiratory center. The same coupling effect may be achieved byactivation of the cortical respiratory center by means other thanvoluntary breathing, such as transcranial magnetic stimulation (TMS) ortranscranial electrical stimulation (TES).

Improved methods and devices are provided for controlling neuromuscularelectrical stimulation by voluntary breathing. The new methods anddevices of breathing-controlled electrical stimulation are based on theintrinsic physiological coupling between the respiratory and motorsystems. The new methods and devices broaden clinical applications ofelectrical stimulation and enhance the effectiveness of electricalstimulation to skeletal muscles. In one example embodiment, the methodsand devices are used for hand rehabilitation in stroke patients based onthe finger flexion-expiration, finger extension-inspiration coupling. Inanother embodiment, the methods and devices are applied to post-strokerehabilitation of other skeletal muscles, such as those in the arms andlegs. In further embodiments, the methods and devices can be used inpatient populations with different neurological disorders, for examplepatients with traumatized brain injury, spinal cord injury, cerebralpalsy, or multiple sclerosis, or in healthy patients for performanceenhancement.

The breathing-controlled electrical stimulation system can be furtherunderstood with reference to the exemplary, non-limiting embodimentsillustrated in FIGS. 1-6.

One embodiment of the breathing-controlled electrical stimulation systemis shown in FIG. 1. The breathing-controlled electrical stimulationsystem 100 triggers the delivery of neuromuscular electrical stimulationbased on the airflow rate during voluntary breathing. The systemreceives a respiratory signal, processes the signal, and based on thesignal triggers the delivery of electrical pulses to target skeletalmuscles. The breathing-controlled electrical stimulation system 100comprises a face mask 102 with attached securing straps 104, an airflowrate monitor 106, a control circuit 108, an electrical stimulationdelivery controller 110, and a plurality of electrodes 112. The systemhas three major operational components: the airflow rate monitor 106,the control circuit 108, and the electrical stimulation deliverycontroller 110. FIG. 2 is a block diagram which provides additionaldetails regarding an example embodiment of each operational component.

The face mask 102 collects the respiratory signal from the patient'sbreathing. The respiratory signal is a physiological signal that isindicative of respiratory activities. The respiratory signal canencompass several respiratory parameters, including respiratory cyclelength, inspiration period, expiration period, non-breathing interval,tidal volume, breathing rate, and airflow rate. The use of a face mask102 to collect a patient's respiratory signal is well known in the art,and the face mask 102 could have a variety of designs so long as itworks to achieve this purpose. Example embodiments of the face mask 102may include pressure transducers, temperature sensors, humidity sensors,gas sensors, such as oxygen or carbon dioxide, or any sensor combinationthereof. The afore-mentioned sensors may be implemented by many knownsensor devices, such as mechanical, strain gauge, piezoelectric,microelectromechanical, pyroelectric, photoelectric, vibrating,capacitance, or optical based sensors, or any combination thereof.

In the embodiment of FIG. 1, the face mask 102 is secured to thepatient's face by securing straps 104. The face mask 102 preferably hasa tight fit against the patient's face. The patient may be allowed tobreathe using both the mouth and nose. Alternatively, depending on thedesign of face mask 102 or other respiratory signal collector, thepatient may be directed to breathe through the mouth or noseexclusively. In other embodiments, the respiratory signal could becollected by a variety of known mechanisms other than a face mask. Inone embodiment, for example, the patient breathes directly from his orher mouth into a tube. Another embodiment includes a nasal tube orcannula, which is inserted into or placed around the patient's nasalpassages and may be clipped to the patient's nose to secure it in place.

In the embodiment of FIG. 1, the face mask 102 is coupled to an airflowrate monitor. The respiratory signal collected by the face mask 102 iscontinuously fed to the airflow rate monitor 106. The airflow ratemonitor 106 processes the respiratory signal to determine the airflowrate of the patient's breathing.

The airflow rate is the main parameter used to differentiate automaticbreathing from voluntary breathing and is used as thetriggering-signaling parameter in the current embodiment. However, otherrespiratory parameters, such as the tidal volume and the respiratorycycle length, also may be used to differentiate automatic breathing fromvoluntary breathing. In certain embodiments, other such parameters maybe substituted as the triggering-signaling parameter, and theoperational components of the system are adjusted accordingly to measureand respond to the other parameter in place of airflow rate. Forexample, in one embodiment the system has a tidal volume monitor insteadof an airflow rate monitor, and the control circuit is designed toprocess and respond to a tidal volume signal threshold. In furtherembodiments, instead of a single trigger-signaling parameter, acombination of respiratory parameters may be used, and again, theoperational components are adjusted accordingly.

FIG. 2 provides an overview of functional details of one exampleembodiment of the airflow monitor 106. The airflow monitor 106 showncomprises a respiratory sensor, a sensor processing circuit, and anairflow rate detector. The respiratory signal passes directly from theface mask 102 to the respiratory sensor. The respiratory sensor receivesthe respiratory signal, including all of the respiratory parametersmentioned above. The respiratory sensor does not process the respiratorysignal, but passes it to the sensor processing circuit. The processingcircuit separates the signal into the various respiratory parameters andinterprets at least one parameter, in this embodiment airflow rate, intoa format readable by the airflow rate detector. Even in an embodiment inwhich the airflow rate is the only trigger-signaling parameter, thesensor processing circuit may interpret other respiratory parameters,although these parameters do not affect the system's functioning. Uponreceipt of the signal input from the processing circuit, the airflowrate detector determines the airflow rate of the patient's breathing.These functions occur quickly and continuously as the patient breathes.

The configuration shown in FIG. 2 is optional; the airflow rate monitor106 can have a variety of configurations known in the art to perform thefunction of receiving a respiratory signal and detecting the airflowrate. The airflow rate monitor 106 may be a combination of hardware andsoftware-based components, which may be implemented, for example, by amicroprocessor, an integrated circuit, a field programmable gate array(“FPGA”), electronic circuitry, or any combination thereof. The airflowrate monitor 106 may further include software-based logic for performingsignal processing on the received respiratory signal, such as performingtransforms, filtering, unit conversion, and the like. In otherembodiments, some or all of the processing functions described as beingperformed may be performed at least partially by the control circuitdescribed below. Further, there is no separation required between theshown functional components of the face mask 102 or other respiratorysignal collector and the airflow rate monitor 106. For example, therespiratory sensor may be a part of the face mask 102 itself, as opposedto the airflow rate monitor 106. The same options apply if the monitoris designed for another respiratory parameter, such as tidal flowvolume, or a combination of respiratory parameters, as discussed above.

The control circuit 108 functions as the central controller of thesystem. Upon processing the respiratory signal to determine the airflowrate, the airflow rate monitor 106 sends a signal identifying theairflow rate to the control circuit 108. Through a control algorithm,the control circuit 108 determines when electrical stimulation occurs.The control algorithm is preferably software-implemented. The controlalgorithm may be implemented by a microprocessor, an integrated circuit,a field programmable gate array (“FPGA”), electronic circuitry, or anycombination thereof, or through other hardware and/or software-baseddevices known to one of skill in the art. A software-implementedmicroprocessor-based control circuit may include a memory that storesprogrammed logic (for example, software). The memory may also includedata utilized in the operation of operating system in some embodiments.For example, a processor may utilize the operating system to execute theprogrammed logic, and in doing so, may also utilize the data, which mayeither be stored data or data obtained through measurements or externalinputs. A data bus may provide communication between the memory and theprocessor. Users may interface with the control circuit 108 via one ormore user interface device(s), such as a keyboard, mouse, control panel,or any other devices suitable for communicating digital data to thecontrol circuit 108. The user interface device(s) may communicatethrough wired communication, which may be removably coupled to the pulsegenerator during implantation or during servicing, or may communicatewirelessly, such as through radio frequency, magnetic, or acoustictelemetry, for example. The control circuit 108 and the programmed logicimplemented thereby may comprise software, hardware, firmware, or anycombination thereof. Although the control circuit 108 is described asbeing implemented by a single controller, multiple control circuits 108may be employed, with each performing individual functions and/or eachperforming redundant functions of the other. Some of the componentsdescribed may exist external to the device(s) which house the airflowrate monitor and/or the electrical stimulation delivery controller, forexample, within a separate processing unit, such as a personal computeror the like, that is in communication with such device(s).

As shown, in this embodiment the control circuit 108 receives real-timeinformation about the airflow rate, which it continuously compares withan adjustable pre-determined airflow rate threshold. When the airflowrate reaches or exceeds the pre-determined threshold, the controlcircuit 108 sends a trigger signal to the electrical stimulationdelivery controller 110, which in turn generates an adjustable degree ofelectrical stimulation and delivers it through the electrodes 112. Thecontrol circuit 108 receives continuous input from the airflow ratemonitor 106 and performs the algorithm continuously, such that a triggercommand is sent by the control circuit 108 every time the presetthreshold is reached. When the detected airflow rate is below thethreshold, no trigger signal is sent by the control circuit 108. Thus,until the airflow rate threshold is reached, or between breaths whichmeet the threshold, breathing continues without the patient receivingneuromuscular electrical stimulation.

The threshold used by the control circuit's algorithm can be determinedin various ways. According to one method, the patient's maximalinspiration airflow rate during voluntary breathing is measured prior tothe procedure and used to set the threshold. The threshold can be chosenas any percentage of the maximum inhaling airflow rate during voluntarybreathing. The desired percentage may vary according to the clinicalapplication. In certain embodiments, the threshold varies from 20percent to 60 percent of the patient's maximum inhaling airflow rateduring voluntary breathing. In a preferred embodiment, the threshold is40 percent of the patient's maximum inhaling airflow rate duringvoluntary breathing. If a respiratory parameter other than airflow rateis used as the trigger signal, then the threshold can be chosen as apercentage of the maximum value of that parameter during voluntarybreathing or otherwise, as appropriate. Although preset, the thresholdcan be adjusted during treatment, as desired.

According to another method, the threshold is arbitrarily set to a valuewithout testing before the procedure. The value can then be manuallyadjusted to deliver electrical stimulation at appropriate points basedon the airflow rate or other parameter of the patient's voluntarybreathing during treatment. The threshold can be further adjusted if therelevant parameter(s) of the patient's breathing changes duringtreatment (for example, as a result of breathing faster, deeper, orslower).

FIG. 3 is an illustration of example respiratory cycles. As shown, theairflow rate of the patient's breathing is measured in liters per second(L/s). Positive values indicate the inspiration phase, while negativevalues indicate the expiration phase. The airflow rate crossing zeroindicates the transition between respiratory phases, from inspiration toexpiration or vice versa. The graph shows three inspirations,represented by positive values on the graph. The second inspiration, themiddle peak, represents voluntary breathing, in particular forcedinspiration. The positive values to the left and right of the middlepeak represent inspirations during automatic breathing. As shown, theairflow rate is significantly larger during voluntary breathing thanduring automatic breathing. The dashed horizontal line represents anexample pre-determined airflow rate threshold which is approximately 50percent of the patient's maximum inhaling airflow rate during voluntarybreathing. The dashed vertical line represents the point in time whenthe real-time airflow rate reaches the threshold. At that point, asdiscussed, the control circuit 108 sends a trigger command to theelectrical stimulation delivery controller 110 to generate an electricalpulse.

The electrical stimulation delivery controller 110 is the execution partof the system. FIG. 2 shows functional details of one embodiment of theelectrical stimulation delivery controller 110. In the embodiment shown,the trigger command from the control circuit 108 is received by theelectrical stimulation generator. Upon receipt of the trigger command,the electrical stimulation generator generates an electrical stimulationpulse. The pulse passes to the stimulation output circuit, whichdelivers the stimulation pulse to the patient's target skeletalmuscle(s) via surface electrodes 112. The configuration shown in FIG. 2is optional; the electrical stimulation delivery controller 110 can havea variety of configurations known in the art to perform the function ofgenerating and delivering electrical stimulation upon receipt of atrigger command. An example electrical stimulation delivery controller110 may include a power source, capacitor circuitry, which may be usedto charge and discharge during defibrillation, and control logic, suchas may be at least partially implemented as a hardware and/orsoftware-based device, for example the control circuit 108 describedherein, as known to one of skill in the art.

Surface electrodes 112 are connected by wires or otherwise to theelectrical stimulation delivery controller 110, in particular thestimulation output circuit in this embodiment. The surface electrodes112 are placed overlying the target muscle belly. Placement of theelectrodes 112 may be adjusted to achieve on maximal isolated responsesfrom the target muscles. The electrodes 112 are preferably adhered tothe surface of the patient's skin using methods known in the art. In oneembodiment, the target muscles are wrist and finger extensor muscles. Inalternative embodiments, the target muscles are other skeletal muscles.In certain embodiments, the target muscles are in the patient's arms andlegs. In particular, the target muscles can include the triceps, thetibialis anterior, and the peroneal muscles. Also, multiple pairs ofsurface electrodes 112 may be configured to deliver neuromuscularelectrical stimulation to multiple target muscles at the same time. Infurther embodiments, instead of surface electrodes 112, the system mayuse implantable electrodes. The implantable electrodes 112 may beconnected to the electrical stimulation delivery controller 110 viadetachable wires or may have a wireless connection.

The electrical stimulation generated upon each trigger command may be asingle pulse or, alternatively, a modified pattern or series ofelectrical stimulation pulses. The characteristics of the electricalpulse or pulses can be preset, and the desired characteristics depend onthe particular clinical application. The frequency of pulses can rangefrom approximately 2 to 100 pulses per second (pps). In a preferredembodiment, the frequency is between 20 and 50 pps. The duration of thepulses can range from approximately 20 to 600 μs. In a preferredembodiment, the duration is between 100 and 300 μs. The maximal peakamplitude of pulses, depending on whether output current or voltage isregulated, does not exceed peak values of 200 mA or 500 V, respectively.Depending on a patient's tolerance, the desired amplitude is selectedduring the treatment when the maximal isolated response is triggeredfrom the target muscle(s), starting from low to high amplitudes.Although values outside of the stated ranges are contemplated by thepresent invention, these ranges will apply for most clinicalapplications.

The frequency, duration, and amplitude can be manually adjusted duringthe treatment, as needed. Other pulse characteristics which may bemodified include the AC/DC status, the waveform, and the on/off time. Ifa series of pulses is generated for each trigger command, thecharacteristics of the pulses within the series can vary. Further, ifthere are multiple sets of electrodes delivering pulses to multiplemuscles, different pulse characteristics may be set for each type ofmuscle. Typically larger skeletal muscles, such as those in the legs,can tolerate greater pulse frequency, duration, and amplitude thansmaller muscles, such as those in the fingers and wrists.

One clinical application of breathing-controlled electrical stimulationis to maximize the effectiveness of neuromuscular electrical stimulationfor stroke rehabilitation. As discussed herein, one example embodimentis to facilitate motor recovery of hand function in stroke patients. Thefeasibility and effectiveness of breathing-controlled electricalstimulation method in this example embodiment has been demonstrated in apilot study with stroke patients.

EXAMPLE 1

Data from one study is shown in FIG. 5, which supports that the mosteffective response to electrical stimulation can be achieved duringforced inspiration. In particular, FIG. 5 shows an example response froma stroke patient when a single electrical pulse is delivered to thefinger extensors during three different breathing conditions. The strokepatient (male, 75 years of age, right CVA/left hemiplegia for 22 years)had intact sensation and weak voluntary finger extension. During theelectrical stimulation treatment, the patient was seated in acomfortable position in a chair with the impaired forearm stabilized inthe neutral position. The patient's wrist was also stabilized in theneutral position using Velcro stripes. The mid-shafts of proximalphalanges were stabilized against four force sensors such that themetacarpophalangeal (MCP) joints were in approximately 30 degrees offlexion. The maximal finger extension force measured at MCP joints was11.1 Newtons. The patient was instructed to maintain a 30% maximal force(represented by the lower dashed line across the force graphs) of hisleft finger extensors by following a visual target on the computerscreen. A face mask was used to collect the patient's respiratorysignal. The maximal airflow rate was measured when the patient wasinstructed to inhale as fast and as hard as possible (forcefulinspiration) and to exhale as fast and as hard as possible (forcefulexpiration). The measured maximal airflow rate was 5.6 L/s for forcefulinspiration and −5.9 L/s for forceful expiration. The airflow ratethreshold was preset as 40% of each measured maximal airflow rate. Forthis patient, the threshold was 2.2 L/s for forceful inspiration and−2.4 L/s for forceful expiration. After maintaining the finger extensionforce following the visual target, the patient was instructed toinitiate one forceful expiration (Expiration) at his convenience duringa 10 second trial, and otherwise to breathe normally. This process wasrepeated with the patient being instructed to initiate one forcefulinspiration (Inspiration) in a second trial. During a third trial, thepatient was instructed to breathe only normally (Norm). The airflow rateduring each of these trials is shown in L/s on the airflow graphs inFIG. 5.

The patient's real-time airflow rate, as shown on the graphs, wascontinuously compared with the preset threshold. A single pulseelectrical stimulation was delivered to the finger extensors when thepreset airflow rate threshold was reached under each of the Expirationand Inspiration conditions. A single pulse electrical stimulation wasrandomly triggered and delivered to the finger extensors under the Normcondition. Each pulse was a square wave with a duration of 0.1 ms and anintensity of 120 Volts. As shown on FIG. 5, a force spike occurred inresponse to the electrical pulse under each of the three conditions. Themagnitude of force response, or force increment, was calculated as thedifference between the force response peak and the mean force averagedover a 50 ms window prior to the electrical stimulation delivery. Thetrials had a similar baseline force, which indicates a similarbackground level of activation under these conditions. The forceresponse during the Inspiration condition is visibly higher on thegraphs than the force response during the Norm and Expirationconditions. The induced force increment, on average, was 5.8 Newtons,6.2 Newtons, and 6.8 Newtons for the Normal, Expiration, and Inspirationconditions, respectively. When normalized to the force increment duringnormal breathing (Norm), the overall increment was 106.6% duringforceful expiration and 116.6% during forceful inspiration. Thus, themost effective response to electrical stimulation was achieved duringforced inspiration.

Although enhanced responses are expected and observed when electricalstimulation is delivered to the finger extensors during forcedexpiration, triggering stimulation based on exhalation should be avoidedin rehabilitation applications, such as motor recovery in strokepatients. Unlike forced inspiration, which facilitates primarily fingerextension, forced expiration facilitates both finger flexion and fingerextension. As a result, synchronizing electrical stimulation with forcedexpiration can lead to problematic coactivation.

EXAMPLE 2

Data from another study is shown in FIG. 6, which supports the positiverehabilitation effect of breathing-controlled electrical stimulationintervention. In particular, FIG. 6 shows example responses from anotherstroke patient before and after a 25 minute breathing-controlledelectrical stimulation intervention. The patient (male, 60 years of age,left CVA/right hemiplegia for 2 years and 2 months) had intact sensationbut was without voluntary finger/wrist extension. The same experimentalset-up was used for this patient as described above in the context ofFIG. 5. The procedure differed from that described above. The patientwas directed to initiate repeated forced inspiration, as opposed to thethree breathing conditions above, and electrical stimulation wasdelivered only under the Inspiration condition. Single electrical pulseswere triggered and delivered to his right finger extensors each time theinspiration airflow rate reached 2.0 L/s, 40% of the patient's maximalinspiration airflow rate of 5.1 L/s. Each electrical pulse was a squarewave with a duration of 0.1 ms and an intensity of 160 Volts. Thisprocedure was carried out for a total of 25 minutes, during whichapproximately 100 stimuli were delivered. Breaks were allowed as neededduring the intervention. To test the effect of the intervention, thepatient's baseline responses were measured immediately before and afterthe 25 minute procedure. Specifically, finger force responses weremeasured when the patient was at rest and when a series of electricalpulses were delivered. The duration and intensity of the electricalpulses were the same as those used during the intervention. The pre- andpost-intervention responses are shown in FIG. 6. As is visible from thegraphs, the stimulation-induced responses approximately doubled afterthe intervention. The patient also reported that his finger flexorspasticity was dramatically reduced and that the spasticity reductionlasted about three days after the intervention. This period of relief issignificantly longer than the relief achieved using other methods ofelectrical stimulation for stroke rehabilitation.

Breathing-controlled electrical stimulation has the followingadvantages: 1) It encourages and requires active engagement of patientssince voluntary breathing is required. Active engagement of patients isassociated with better results. 2) It eliminates the problems associatedwith obtaining a “clean” EMG signal because no EMG signals are required.Electrical stimulation is triggered and delivered to the fingerextensors when a certain breathing (inspiration) threshold, instead ofEMG threshold, is reached. 3) It has broader clinical applications.Based on the intrinsic physiological coupling, breathing-controlledelectrical stimulation can be applied to patients with moderate tosevere impairment of finger extension, who are not candidates forEMG-triggered stimulation because they are not able to generatesufficient movement to reach the EMG muscle activity threshold. 4) Itreduces the coactivation problem. The co-contraction between fingerflexors and extensors and finger flexor hypertonia associated with usingEMG-triggered electrical stimulation in stroke patients would occur onlyminimally with breathing-controlled electrical stimulation becauseelectrical stimulation that is synchronized with forced inspirationenhances primarily activation of the finger extensors. 5) It results inlong-lasting reduction of finger (wrist) flexor hypertonia. Throughreciprocal inhibition mechanisms, electrical stimulation applied duringthe inspiration phase can inhibit activation of flexor muscles, in turnleading to reduction of flexor hypertonia in stroke patients. Theadvantage of using forced inspiration to trigger electrical stimulationis signified in the pilot stroke data shown in FIG. 6. Baselineresponses to electrical stimulation in a patient without voluntarywrist/finger extension increased (approximately doubled) after a 25minute intervention session. Such results have not been possible withother forms of rehabilitation. For the foregoing reasons,breathing-controlled electrical stimulation is a better choice ofintervention for post-stroke motor recovery of finger extensionimpairments.

Breathing-controlled electrical stimulation can be used to similarlyenhance the effect of neuromuscular electrical stimulation in a widerange of applications. For post-stroke patients, the device can be usedto aid in the rehabilitation of any skeletal muscles other than thefinger/wrist extensors. Breathing-controlled electrical stimulation canalso be used in patients with different neurological disorders,including traumatic brain injury (TBI), spinal cord injury (SCI),cerebral palsy, or multiple sclerosis. The effectiveness ofbreathing-controlled electrical stimulation in these applications isexpected, given the system's usage of respiratory-motor coupling, andsuch effectiveness has been supported.

EXAMPLE 3

A pilot study with a chronic traumatized brain injury (TBI) patientdemonstrated significant spasticity reduction. The patient (male, 27years of age, left hemiplegia resulted from TBI for 13 years) had intactsensation of the left forearm and hand and no voluntary wrist/fingerextension. The Modified Ashworth Scale of left finger flexors was 3 (0:normal, 4: fixed posture due to hypertonia). The patient receivedapproximately 25 min of breathing-controlled electrical stimulationusing the same protocol as described in Example 2. The intensity ofelectrical stimulation was 140 V. The duration of pulse was 0.1 ms. Thepreset inspiration airflow rate was 2.0 L/s, equivalent to 50% of themeasured maximal inspiration airflow rate (4.0 L/s). The patientreported loosening of his left finger flexors immediately after thetreatment, and the Modified Ashworth Scale was 2. This significantreduction in finger flexor hypertonia was retained until the end of4-week follow-up from his treatment.

Breathing-controlled electrical stimulation has also been shown by twostudies (Li et al. 2007, and Sieler et al. 2008) to strengthen skeletalmuscles for performance enhancement for healthy individuals.

EXAMPLE 4

In one pilot study (Li et al. 2007), using the experimental protocoldescribed in Example 1, the force responses to electrical stimulation offinger extensors were measured under different breathing conditions.Three young and healthy males were instructed to produce constantisometric finger extension forces at 10%, 20%, and 30% of their maximalforce, respectively. Electrical stimulation was delivered to the fingerextensors during normal breathing, forced inspiration (IN) and forcedexpiration (OUT). The force increment, or force response to electricalstimulation, was normalized to that measured during normal breathing.The normalized increment increased considerably and consistently acrossall tested force levels. On average, the overall normalized incrementwas 153.2% during OUT and 143.2% during IN.

EXAMPLE 5

In another pilot study (Sieler et al. 2008), three young and healthyfemales were instructed to produce constant isometric finger extensionforce at 10% of their maximal force or at rest. The same protocol wasused as in Example 4. The normalized increment increased consistently inall three subjects both at 10% and at rest. The average normalizedincreased was 120.2% during OUT and 115.8% during IN.

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1. A neuromuscular electrical stimulation device, comprising: a. arespiratory signal collector adapted to receive a respiratory signal; b.a respiratory signal monitor adapted to interpret the respiratory signalto detect the value of at least one respiratory parameter; c. a controlcircuit adapted to compare the detected value of the at least onerespiratory parameter value to a preset threshold value, further whereinthe control circuit sends a trigger command when the detected valuereaches the threshold value; and d. an electrical stimulation deliverycontroller adapted generate electrical stimulation upon receipt of thetrigger command from the control circuit.
 2. The device of claim 1,wherein the detected respiratory parameter can differentiate voluntarybreathing from autonomic breathing.
 3. The device of claim 1, whereinthe electrical stimulation delivery controller generates a singleelectrical pulse upon receipt of each trigger command.
 4. The device ofclaim 1, wherein the electrical stimulation delivery controllergenerates a series of electrical pulses upon receipt of each triggercommand.
 5. The device of claim 1, wherein airflow rate is the detectedrespiratory parameter.
 6. The device of claim 5, wherein the respiratorysignal collector is selected from the group consisting of a face mask, anasal tube, and a mouth tube.
 7. The device of claim 5, furthercomprising surface electrodes adapted to deliver electrical stimulationto a patient's skeletal muscles.
 8. The device of claim 7, wherein theelectrical stimulation is delivered to muscles in a patient's arm(s) orleg(s).
 9. The device of claim 7, wherein the electrical stimulation isdelivered to muscles in a patient's finger(s) or wrist(s).
 10. Thedevice of claim 1, wherein airflow rate is one of a plurality ofdetected respiratory parameters.
 11. The device of claim 1, furthercomprising implantable electrodes adapted to deliver electricalstimulation to a patient's skeletal muscles.
 12. A method ofbreathing-controlled electrical stimulation, comprising: a. collecting apatient's respiratory signal; b. interpreting the respiratory signal todetect at least one respiratory parameter capable of differentiatingvoluntary breathing from autonomic breathing; and c. deliveringelectrical stimulation to a patient's target muscles when voluntarybreathing is detected.
 13. The method of claim 12, wherein airflow rateis the detected respiratory parameter.
 14. The method of claim 12,wherein electrical stimulation is delivered only when forced inspirationis detected.
 15. The method of claim 14, wherein the method is used forpost-stroke rehabilitation.
 16. The method of claim 15, wherein thepatient's target muscles are in the wrist or finger.
 17. The method ofclaim 12, wherein the patient's target muscles are any skeletal musclesin the arm or leg.
 18. The method of claim 12, wherein the applicationof the method is selected from the group consisting of rehabilitationfollowing a stroke, rehabilitation following a spinal cord injury,rehabilitation following a traumatic brain injury, rehabilitation ofpatients with cerebral palsy, and rehabilitation of patients withmultiple sclerosis.
 19. The method of claim 12, wherein the method isused for performance enhancement in healthy patients.
 20. The method ofclaim 12, wherein electrical stimulation is delivered to the patient'smuscles through surface electrodes.
 21. A method of breathing-controlledelectrical stimulation, comprising: a. collecting a patient'srespiratory signal; b. interpreting the respiratory signal to detect thevalue of at least one respiratory parameter which is capable ofdifferentiating voluntary breathing from autonomic breathing; c.comparing the detected value of the at least one respiratory parameterwith a preset threshold value; and d. delivering electrical stimulationto a patient's muscles when the detected value meets the presetthreshold value.
 22. The method of claim 21, wherein airflow rate is thedetected respiratory parameter.
 23. The method of claim 21, wherein thepreset respiratory parameter threshold is adapted to correspond toforced inspiration.
 24. The method of claim 23, wherein the method isused for post-stroke rehabilitation.
 25. The method of claim 24, whereinthe patient's target muscles are in the wrist or finger.
 26. The methodof claim 21, wherein the patient's target muscles are any skeletalmuscles in the arm or leg.
 27. The method of claim 21, wherein theapplication of the method is selected from the group consisting ofrehabilitation following a stroke, rehabilitation following a spinalcord injury, rehabilitation following a traumatic brain injury,rehabilitation of patients with cerebral palsy, and rehabilitation ofpatients with multiple sclerosis.
 28. The method of claim 21, whereinthe method is used for performance enhancement in healthy patients. 29.The method of claim 21, wherein electrical stimulation is delivered tothe patient's muscles through surface electrodes.